Page 115 - 86 human physiology part-2

Chapter 17

4 Weeks

• There are only the beginnings of facial features. All the major organs are starting to form.
Gill-like folds that develop into facial features, beginnings of the spinal cord, skin is translucent,
and rudimentary (basic; minimal) heart develops.

6 Weeks

• The length from crown to rump is about the size of a finger tip, ¾ ". The beginnings of all
the major organs will have formed.
• The embryo floats in a fluid filled bubble that will develop into the amniotic sac. The sac is
covered by a protective layer of cells, called chorion. The yolk sac supplies the embryo with all
its nutrients until the placenta is fully developed and takes over at around the twelfth week.
During the first 12 weeks, the embryo will develop features and major organs of a human being.
The embryo is susceptible to harmful environmental influences. This is a vital time for the
embryo to develop healthily; taking supplements of folic acid, avoiding certain foods, and
eliminating alcohol, cigarettes, and any unnecessary drugs or medicines.

9 Weeks

• The length from crown to rump approximately 1 1/4". The facial features are becoming
more distinct, and the “tail” has disappeared. The muscles are also developing. Eyes are formed
but eyelids are still closed over them. Arms now bend at the elbow and rudimentary hands and
fingers develop. Knees will have formed and developing feet with distinct toes.
• Heart- is now a four-chambered and fully formed organ; it beats about 180 times per
minute.
• Brain and nervous system- is four times the size it was at 6 weeks. Special glial cells are
being formed within the neural tube; they allow nerve cells to be joined so that messages can be
transmitted from the brain to the body.
• Digestive system- the mouth, intestine, and stomach are developing very rapidly, but do not
function yet.
• The fetal life-support system- the placental tissue that initially surrounds the fetus and the
amniotic sac is becoming concentrated in one circular area on the womb wall to form the
placenta.

12 Weeks

• At twelve weeks the fetus looks like a tiny human. It is about 2 1/2" long and weighs 1/2
oz. Arms and legs are now beginning to move. Skin is red and translucent. Fingers and toes are
more defined, and nails are starting to grow.
• Heart is complete and working, pumping blood to all parts of the body. Digestive system
has formed and is linked to the mouth and intestines. Sexual organs have formed inside the
body, but cannot yet establish the sex of the baby.

330 | Human Physiology